The present invention relates to a scanning optical system, and more particularly to a scanning optical system employed in a tandem type color laser printer or the like, for scanning a plurality of beams simultaneously.
Conventionally, a tandem type color laser printer has been known. The tandem type color laser printer typically has four photoconductive drums corresponding to four colors of Y (yellow), M (magenta), C (cyan) and B (black). In such a printer, a recording sheet is fed in one direction, and images of respective color components are transferred on the recording sheet sequentially, to form a color image. A scanning optical system for this type of color printer, four laser diodes corresponding to the four colors are provided. Four laser beams emitted by the four laser diodes are deflected by a single common polygonal mirror. The deflected beams are incident on a common fθ lens, which converges the beams on the photoconductive drums, respectively. Typically, compensation lenses that compensate for curvature of field are provided in the vicinity of respective photoconductive drums.
With the above configuration, each of the beam spots formed on the respective photoconductive drums scans as the polygonal mirror rotates. At the same time, the photoconductive drums are rotated. By ON-OFF modulating the beams while they scan on the photoconductive drums, two-dimensional latent images are formed on the photoconductive drums, respectively. The latent images are developed by applying toners of respective colors, and then transfer the developed images on the recording sheet, a color image is formed, and fixed.
In this specification, a main scanning direction and an auxiliary scanning direction will be referred to as follows. With the optical paths on the downstream side of the polygonal mirror being considered to be developed, a direction in which the beam spot scans (moves) on the photoconductive drum (a surface to be scanned) will be referred to as the main scanning direction, and a direction parallel with a rotation axis of the polygonal mirror (i.e., a direction perpendicular to the main scanning direction and on a plane including the optical axis of the scanning lens) will be referred to as the auxiliary scanning direction. Shape and/or power of each element will be described with reference to the main and auxiliary scanning directions on the photoconductive drum.
When a single polygonal mirror is used for deflecting a plurality of beams, by differentiating incident angles of the beams with respect to the polygonal mirror in the auxiliary scanning direction, it becomes possible that all the beams are incident on substantially the same point on the polygonal mirror. With such a configuration, the thickness of the polygonal mirror can be reduced, which lowering manufacturing costs of the polygonal mirror.
When a beam incident on the polygonal mirror has a certain incident angle, the bows of the scanning lines corresponding to the beams have different shapes. In the tandem type color laser beam printer, if the scanning lines have different shapes, color shift occurs in the finally obtained color image. Therefore, it is very important to have similarity in the shapes of the bows.
In the conventional scanning optical system, however, for the beams incident on different incident angles, different compensation lenses for compensating the curvature of field are used. Such lenses are designed so as provide optimum performances to compensate for the curvature of field with respect to the beam passing therethrough. Regarding bows, the scanning optical system may be designed so that the bows are minimized. However, if bows appear due to, for example, assembling errors or the like, since the compensation lenses are not designed to compensate for the bows, the shapes of the bows may be different from each other.
Even if the bows occur due the assembling error or the like, if they have the same shapes, by rotating each compensation lens about an axis extending in the main scanning direction, it is possible to make effects of the bows on the corresponding images identical. If this can be done, even if the bows remain, by appropriately adjusting image signal, the effects of the bows can be removed. However, in the conventional tandem type color laser printer, the shape of bows are different from each other, and therefore, even if the compensation lens for the curvature of field is rotated, the shapes of the bows cannot be made identical.